This invention relates generally to shadowgraph imaging apparatus, and, more particularly, to shadowgraph apparatus for detecting defects in recording disks. In general terms, a shadowgraph is an optical image formed by shadows cast on a screen when a source of radiation is directed onto the screen through an object being examined. In this broad sense, x-ray pictures are shadowgraphs. Shadowgraphs are also employed to study the characteristics of fluid flow around objects such as aircraft, ballistic devices, and so forth. Light passing through the moving fluid is used to produce a shadow image caused by refraction through density gradients in the fluid. This technique is commonly referred to as the schlieren method.
In video disc recording technology, video information is recorded on a master disc, from which a great many copies are subsequently made for sale to the general public. Typically, the disc copies are formed in a plastics molding machine. In one process, each face of a finished disk is formed separately as a transparent disc-shaped element, then the two separate elements are bonded together and further processed to form the finished disc. Reproduction of information from the disk may be effected optically, using a laser beam, or may require the use of a transducer that senses changes in some electrical characteristic. In any event, the information is carried on the disc in the form of surface discontinuities that can be detected optically or electrically. Apart from these discontinuities, the surface must be completely clear of optical aberrations that could effect the quality of the reproduced signal. In the past, disc inspection has been carried out by viewing the transparent disc element against a white light source. Although this technique is satisfactory for detecting serious defects in the discs, small anomalies undetectable to the human eye can still create significant problems when the recorded information is played back from the disc.
Accordingly, since the introduction of video disc recording systems, requiring the production of discs to a high degree of accuracy, there has been a significant need for a quality control technique to ensure that manufactured discs are free of anomalies that could affect the quality of reproduction. The present invention is directed to this end.